The current trend in the sector of mobile phone manufacturers, and more generally handheld device manufacturers, is to incorporate added value wireless services, such as connectivity functionality and geolocalization (such as for example, but not limited to Bluetooth™, IEEE802.11a, IEEE802.11b, IEEE802.11g, WLAN, WiFi, UWB, ZigBee, GPS, Galileo, SDARs, XDARS, WiMAX, DAB, FM, DVB-H, or DMB) in more and more of their products. An antenna arranged or configured to operate effectively in a frequency band suitable for one or more of these services or standards is sometimes referred to as a “wireless connectivity antenna” in this document.
In some cases, these handheld devices also operate in at least one frequency band used for mobile communication services, such as GSM (GSM850, GSM900, GSM1800, American GSM or PCS1900, GSM450), UMTS, WCDMA, or CDMA, apart from having the ability to operate in the frequency band corresponding to the wireless connectivity service.
Although it is possible to integrate all the operating bands of a particular handheld device in a single antenna, the trend in the handset manufacturing industry shows that it is preferred to have two separate antennas: A first antenna is used for the bands of the selected mobile communication services (such as, for example, GSM), and a second antenna is used to allow the device to operate at an additional communication service (such as, for instance, UMTS) or at the frequency bands of a wireless connectivity standard (such as, for example, WLAN or Bluetooth™).
Using two separate antennas presents some advantages:                It can make the design of each one of the antennas easier, as a single multiband antenna covering all the bands of operation of the handset would require a more complicated design.        It also simplifies the radiofrequency (RF) front-end for each one of the two antennas. If there is only one single antenna, the RF front-end to which the antenna is connected would have to include a diplexer or multiplexer capable of separating the frequency bands corresponding to the different services, for example, separating the mobile communication frequency bands from the wireless connectivity frequency bands.        Moreover, it offers more flexibility in the design of the PCB that carries the electronic components and circuitry of the handset or handheld device. In many cases, a PCB designer will preferably lay out the module or chipset providing the wireless connectivity functionality, and the module or chipset for the mobile communication services, in different parts of the PCB.        
The first antenna can typically be, for instance and without limitation, a monopole antenna, an inverted-F antenna (IFA), a patch antenna, or a planar inverted-F antenna (PIFA). Some known solutions for said second antenna include antennas printed on the PCB of the device (such as, for example, but not limited to, a printed IFA), or an antenna component, or a chip antenna.
However, the integration in a handset of a second antenna dedicated to the wireless connectivity services is not trivial. As the space available on the PCB of the device is scarce, antenna solutions with small footprints are advantageous. Printed antennas are typically not small in size, since their dimensions are approximately a quarter of an operating wavelength of the antenna. Chip antennas may achieve some degree of miniaturization (for instance, by loading the antenna with a material with high dielectric constant), however, in many cases, they exhibit poor matching levels, and limited bandwidth, efficiency and/or gain.
One additional problem that further complicates the integration of the wireless connectivity antenna in a handset or handheld device is the low isolation that is usually obtained between this antenna and the antenna used for mobile communications.
Interband isolation can be improved by separating the two antennas further apart, although this might not be practical in typical handsets due to their small size and due to the limited positions that are available to integrate the wireless connectivity antenna. This is the case especially for more recent handset topologies, like for example flip-type (also known as clamshell) phones and slider-type phones (as the one schematically illustrated in FIG. 1). As an alternative, a filter can be used to achieve the required level of isolation between antennas within the operating bands. However, this approach implies adding extra components on the PCB, thus using up more space on the PCB of the device, and resulting in an increase in cost of the handset.
A conventional handset that includes an antenna for mobile communications and an antenna for wireless connectivity is depicted in FIG. 2. For this example, the handset has been selected to have a slider-type topology as schematically illustrated in FIG. 1. A slider-type handset comprises typically a first PCB (100) placed substantially above and parallel to a second PCB (102). The first PCB (100) has the ability to slide above the second PCB (102), so that the handset can be in a closed position, as shown in FIG. 1(a) or in an open position, as shown in FIG. 1(b). Generally, the first PCB (100) and the second PCB (102) are electrically connected, for example by means of a flexible conductive film (not illustrated in FIG. 1). An antenna 101 is mounted at one end of the first PCB. In this case, the antenna is a Planar Inverted-F Antenna (PIFA), with a short-circuit (101B) to the ground-plane (in this case, to a conductive metal layer of the PCB) and with a feeding point (101A) close to said short-circuit.
For the purpose of the example illustrated in FIG. 2, the handset comprises a first antenna (201), placed on the top part of the first PCB (200), that operates at the frequency bands for mobile communications, and a second antenna (202) placed on the bottom right corner of the PCB (200) that operates at the frequency bands of the wireless connectivity services. The first antenna has a feeding point (201A) and a short-circuit (201B) to ground (namely, to a conductive metal layer of the PCB 200, constituting a ground-plane for the antenna). For illustrative purposes, in the example illustrated in FIG. 2, the second antenna (202) is a surface mount technology (SMT) component mounted on the PCB (200), although it could have been replaced by, for example, an antenna printed on the PCB (200).
Some typical electrical results for the handset of FIG. 2 are shown without any limiting purpose in FIG. 3. FIG. 3a presents typical results of the input parameters of the antennas (i.e., return losses of each antenna, and isolation between antennas) when the slider-type phone is in the closed position, while FIG. 3b presents the typical results of the antennas when the phone is in the open position. In this example, the first antenna (201) was designed to have a multiband behavior, with a first resonance around 900 MHz to provide coverage for the GSM900 service, and a second resonance around 1900 MHz to provide service to the GSM1800 and GSM1900 services. On the other hand, the second antenna was designed to be tuned in the 2500 MHz band. These frequency ranges have been selected just to illustrate the example, but the antennas could work in any frequency band included in the range from approximately 400 MHz to approximately 12 GHz, including any subinterval. The isolation between the first antenna (201) and the second antenna (202) is 20 dB in the 900 MHz band, 18 dB in the 1900 MHz band. The isolation degrades to 17 dB at the center of the resonance of the second antenna around 2600 MHz.
Space Filling Curves
In some embodiments of the invention, at least one antenna of the antennas included in the handset or handheld device may be miniaturized by shaping at least a portion of the conducting trace, conducting wire or contour of a conducting sheet of the antenna (e.g., a part of the arms of a dipole, the perimeter of the patch of a patch antenna, the slot in a slot antenna, the loop perimeter in a loop antenna, or other portions of the antenna) as a space-filling curve (SFC).
An SFC is a curve that is large in terms of physical length but small in terms of the area in which the curve can be included. More precisely, for the purposes of this patent document, an SFC is defined as follows: a curve having at least five segments, or identifiable sections, that are connected in such a way that each segment forms an angle with any adjacent segments, such that no pair of adjacent segments defines a larger straight segment. In addition, an SFC does not intersect with itself at any point except possibly the initial and final point (that is, the whole curve can be arranged as a closed curve or loop, but none of the lesser parts of the curve form a closed curve or loop).
A space-filling curve can be fitted over a flat or curved surface, and due to the angles between segments, the physical length of the curve is larger than that of any straight line that can be fitted in the same area (surface) as the space-filling curve. Additionally, to shape the structure of a miniature antenna, the segments of the SFCs should be shorter than at least one fifth of the free-space operating wavelength, and possibly shorter than one tenth of the free-space operating wavelength. The space-filling curve should include at least five segments in order to provide some antenna size reduction, however a larger number of segments may be used. In general, the larger the number of segments and the narrower the angles between them, the smaller the size of the final antenna.
Box-Counting Curves
In other embodiments of the invention, at least one antenna of the antennas included in the handset or handheld device may be miniaturized by shaping at least a portion of the conducting trace, conducting wire or contour of a conducting sheet of the antenna to have a selected box-counting dimension.
For a given geometry lying on a surface, the box-counting dimension is computed as follows. First, a grid with substantially square identical cells or boxes of size L1 is placed over the geometry, such that the grid completely covers the geometry, that is, no part of the curve is out of the grid. The number of boxes N1 that include at least a point of the geometry are then counted. Second, a grid with boxes of size L2 (L2 being smaller than L1) is also placed over the geometry, such that the grid completely covers the geometry, and the number of boxes N2 that include at least a point of the geometry are counted. The box-counting dimension D is then computed as:
  D  =      -                            log          ⁡                      (                          N              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          N              ⁢                                                          ⁢              1                        )                                                log          ⁡                      (                          L              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          L              ⁢                                                          ⁢              1                        )                              
For the purposes of the antennas included in the handset or handheld device described herein, the box-counting dimension may be computed by placing the first and second grids inside a minimum rectangular area enclosing the conducting trace, conducting wire or contour of a conducting sheet of the antenna and applying the above algorithm. The first grid should be chosen such that the rectangular area is meshed in an array of at least 5×5 boxes or cells, and the second grid should be chosen such that L2=½L and such that the second grid includes at least 10×10 boxes. The minimum rectangular area is an area in which there is not an entire row or column on the perimeter of the grid that does not contain any piece of the curve.
The desired box-counting dimension for the curve may be selected to achieve a desired amount of miniaturization. The box-counting dimension should be larger than 1.1 in order to achieve a substantial antenna size reduction. If a larger degree of miniaturization is desired, then a larger box-counting dimension may be selected, such as a box-counting dimension ranging from 1.5 to 3. For the purposes of this patent document, curves in which at least a portion of the geometry of the curve has a box-counting dimension larger than 1.1 are referred to as box-counting curves.
For very small antennas, for example antennas that fit within a rectangle the longest side of which does not exceed one-twentieth the longest free-space operating wavelength of the antenna, the box-counting dimension may be computed using a finer grid. In such a case, the first grid may include a mesh of 10×10 equal cells, and the second grid may include a mesh of 20×20 equal cells. The box-counting dimension (D) may then be calculated using the above equation.
In general, for a given resonant frequency of the antenna, the larger the box-counting dimension, the higher the degree of miniaturization that will be achieved by the antenna. One way to enhance the miniaturization capabilities of the antenna is to arrange the several segments of the curve of the antenna pattern in such a way that the curve intersects at least one point of at least 14 boxes of the first grid with 5×5 boxes or cells enclosing the curve. If a higher degree of miniaturization is desired, then the curve may be arranged to cross at least one of the boxes twice within the 5×5 grid, that is, the curve may include two non-adjacent portions inside at least one of the cells or boxes of the grid.
FIG. 17 illustrates an example of how the box-counting dimension of a curve (1700) is calculated. The example curve (1700) is placed under a 5×5 grid (1701) and under a 10×10 grid (1702). As illustrated, the curve (1700) touches N1=25 boxes in the 5×5 grid (1701) and touches N2=78 boxes in the 10×10 grid (1702). In this case, the size of the boxes in the 5×5 grid (1701) is twice the size of the boxes in the 10×10 grid (1702). By applying the above equation, the box-counting dimension of the example curve (1700) may be calculated as D=1.6415. In addition, further miniaturization is achieved in this example because the curve (1700) crosses more than 14 of the 25 boxes in grid (1701), and also crosses at least one box twice, that is, at least one box contains two non-adjacent segments of the curve. More specifically, the curve (1700) in the illustrated example crosses twice in 13 boxes out of the 25 boxes.
Grid Dimension Curves
In some embodiments of the invention, at least one antenna of the antennas included in the handset or handheld device may be miniaturized by shaping at least a portion of the conducting trace, conducting wire or contour of a conducting sheet of the antenna to include a grid dimension curve.
For a given geometry lying on a planar or curved surface, the grid dimension of curve may be calculated as follows. First, a grid with substantially identical cells of size L1 is placed over the geometry of the curve, such that the grid completely covers the geometry, and the number of cells N1 that include at least a point of the geometry are counted. Second, a grid with cells of size L2 (L2 being smaller than L1) is also placed over the geometry, such that the grid completely covers the geometry, and the number of cells N2 that include at least a point of the geometry are counted again. The grid dimension D (sometimes also referred to as Dg) is then computed as:
  D  =      -                            log          ⁡                      (                          N              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          N              ⁢                                                          ⁢              1                        )                                                log          ⁡                      (                          L              ⁢                                                          ⁢              2                        )                          -                  log          ⁡                      (                          L              ⁢                                                          ⁢              1                        )                              
For the purposes of the antennas included in the handset or handheld device described herein, the grid dimension may be calculated by placing the first and second grids inside the minimum rectangular area enclosing the curve of the antenna and applying the above algorithm. The minimum rectangular area is an area in which there is not an entire row or column on the perimeter of the grid that does not contain any piece of the curve.
The first grid may, for example, be chosen such that the rectangular area is meshed in an array of at least 25 substantially equal cells. The second grid may, for example, be chosen such that each cell of the first grid is divided in 4 equal cells, such that the size of the new cells is L2=½L1, and the second grid includes at least 100 cells.
The desired grid dimension for the curve may be selected to achieve a desired amount of miniaturization. The grid dimension should be larger than 1 in order to achieve some antenna size reduction. If a larger degree of miniaturization is desired, then a larger grid dimension may be selected, such as a grid dimension ranging from 1.5-3 (e.g., in case of volumetric structures). In some examples, a curve having a grid dimension of about 2 may be desired. For the purposes of this patent document, a curve having a grid dimension larger than 1 is referred to as a grid dimension curve.
In general, for a given resonant frequency of the antenna, the larger the grid dimension, the higher the degree of miniaturization that will be achieved by the antenna. One example way of enhancing the miniaturization capabilities of the antenna is to arrange the several segments of the curve of the antenna pattern in such a way that the curve intersects at least one point of at least 50% of the cells of the first grid with at least 25 cells enclosing the curve. In another example, a high degree of miniaturization may be achieved by arranging the antenna such that the curve crosses at least one of the cells twice within the 25-cell grid, that is, the curve includes two non-adjacent portions inside at least one of the cells or cells of the grid.
FIG. 18 shows an example of a two-dimensional antenna forming a grid dimension curve 1800 with a grid dimension of approximately two. FIG. 19 shows the antenna of FIG. 18 enclosed in a first grid 1900 having thirty-two (32) square cells, each with a length L1. FIG. 20 shows the same antenna enclosed in a second grid 2000 having one hundred twenty-eight (128) square cells, each with a length L2. The length (L1) of each square cell in the first grid is twice the length (L2) of each square cell in the second grid (L1=2×L2). An examination of FIG. 19 and FIG. 20 reveals that at least a portion of the antenna is enclosed within every square cell in both the first and second grids. Therefore, the value of N1 in the above grid dimension (D, sometimes also referred to as Dg) equation is thirty-two (32) (i.e., the total number of cells in the first grid), and the value of N2 is one hundred twenty-eight (128) (i.e., the total number of cells in the second grid). Using the above equation, the grid dimension of the antenna may be calculated as follows:
      D    g    =            -                                    log            ⁡                          (              128              )                                -                      log            ⁡                          (              32              )                                                            log            ⁡                          (                              2                ×                L                ⁢                                                                  ⁢                1                            )                                -                      log            ⁡                          (                              L                ⁢                                                                  ⁢                1                            )                                            =    2  
For a more accurate calculation of the grid dimension, the number of square cells may be increased up to a maximum amount. The maximum number of cells in a grid is dependent upon the resolution of the curve. As the number of cells approaches the maximum, the grid dimension calculation becomes more accurate. If a grid having more than the maximum number of cells is selected, however, then the accuracy of the grid dimension calculation begins to decrease. Typically, the maximum number of cells in a grid is one thousand (1000).
For example, FIG. 21 shows the same antenna as that of FIG. 18 enclosed in a third grid 2100 with five hundred twelve (512) square cells, each having a length L3. The length (L3) of the cells in the third grid is one half the length (L2) of the cells in the second grid, shown in FIG. 20. As noted above, a portion of the antenna is enclosed within every square cell in the second grid, thus the value of N for the second grid is one hundred twenty-eight (128). An examination of FIG. 21, however, reveals that the antenna is enclosed within only five hundred nine (509) of the five hundred twelve (512) cells of the third grid. Therefore, the value of N for the third grid is five hundred nine (509). Using FIG. 20 and FIG. 21, a more accurate value for the grid dimension (Dg) of the antenna may be calculated as follows:
      D    g    =            -                                    log            ⁡                          (              509              )                                -                      log            ⁡                          (              128              )                                                            log            ⁡                          (                              2                ×                L                ⁢                                                                  ⁢                2                            )                                -                      log            ⁡                          (                              L                ⁢                                                                  ⁢                2                            )                                            ≈    1.9915  Multilevel Structures
In some examples, at least a portion of the conducting trace, conducting wire or conducting sheet of at least one antenna of the antennas included in the handset or handheld device may be coupled, either through direct contact or electromagnetic coupling, to a conducting surface, such as a conducting polygonal or multilevel surface. A multilevel structure is formed by gathering several geometrical elements, such as polygons or polyhedrons of the same type (e.g., triangles, parallelepipeds, pentagons, hexagons, circles or ellipses—in this context, circles and ellipses are considered to be polygons with a large number of sides—, as well as tetrahedral, hexahedra, prisms, dodecahedra, etc.) and coupling electromagnetically at least some of such geometrical elements to one or more other elements, whether by proximity or by direct contact between elements. The majority of the elements forming part of a multilevel structure have more than 50% of their perimeter (for polygon and surface like elements) not in contact with any of the other elements of the structure. Thus, the elements of a multilevel structure may typically be identified and distinguished, presenting at least two levels of detail: that of the overall structure and that of the polygon or polyhedron elements that form it.
Additionally, several multilevel structures may be grouped and coupled electromagnetically to each other to form higher-level structures. In a single multilevel structure, all of the component elements are polygons with the same number of sides or are polyhedrons with the same number of faces. However, this characteristic is not present when several multilevel structures of different natures are grouped and electromagnetically coupled to form meta-structures of a higher level.
A multilevel antenna includes at least two levels of detail in the body of the antenna: that of the overall structure and that of the majority of the elements (polygons or polyhedrons) which make it up. This may be achieved by ensuring that the area of contact or intersection (if it exists) between the majority of the elements forming the antenna is only a fraction of the perimeter or surrounding area of said polygons or polyhedrons.
One property of multilevel antennae is that the radioelectric behavior of the antenna can be similar in more than one frequency band. Antenna input parameters (e.g., impedance and radiation pattern) remain similar for several frequency bands (i.e., the antenna has the same level of adaptation or standing wave relationship in each different band), and often the antenna presents almost identical radiation diagrams at different frequencies. The number of frequency bands is proportional to the number of scales or sizes of the polygonal elements or similar sets in which they are grouped contained in the geometry of the main radiating element.
In addition to their multiband behavior, multilevel structure antennae may have a smaller than usual size as compared to other antennae of a simpler structure (such as those consisting of a single polygon or polyhedron). Additionally, the edge-rich and discontinuity-rich structure of a multilevel antenna may enhance the radiation process, relatively increasing the radiation resistance of the antenna and reducing the quality factor Q (i.e., increasing its bandwidth).
A multilevel antenna structure may be used in many antenna configurations, such as dipoles, monopoles, patch or microstrip antennae, coplanar antennae, reflector antennae, wound antennae, antenna arrays, or other antenna configurations. In addition, multilevel antenna structures may be formed using many manufacturing techniques, such as printing on a dielectric substrate by photolithography (printed circuit technique); dieing on metal plate, repulsion on dielectric, or others.